This invention relates to phototransistors.
More particularly, the present invention relates to edge illuminated heterojunction bipolar phototransistors (HBPTs).
As the bit rates of telecommunication and datacom systems increase, the demands on the performance requirements of photoreceivers increase. Particularly at bit rates of 10 Gbit/s and 40 Gbit/s, the design of the photoreceiver to achieve these high speeds relies on the ability to reduce the size of the circuit elements and to reduce the parasitic capacitances and inductances throughout the circuit. In typical front-end receivers, some form of photodetector, usually a pin diode, avalanche photodetector, or metal-semiconductor-metal detector needs to be used to convert the incident light pulses into electrical signals. A subsequent transimpedance amplifier then amplifies the electrical signal output of the detector. The output signal needs to be strong so that it can be further processed by a clock and data recovery circuit. For high speeds, it is critical to minimize the internal capacitances and resistances of the aforementioned photodetector. It is also critical to minimize the parasitic capacitances and inductances that arise due to the need to connect the photodetector to the input of the transimpedance amplifier (TIA). In particular, the parasitic inductance associated with this interconnection can significantly impact the overall performance of the front-end receiver by reducing the transimpedance gain and causing circuit instability.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to provide a new and improved heterojunction bipolar transistor optoelectronic transimpedance amplifier using the first transistor as an optical detector.
It is an object of the present invention to provide a new and improved heterojunction bipolar transistor optoelectronic transimpedance amplifier which decreases the excessive parasitic losses associated with having external electrical contacts.
It is another object of the present invention to provide a new and improved heterojunction bipolar transistor optoelectronic transimpedance amplifier which is monolithically integrated on a semiconductor substrate.
A further object of the invention is to provide a new and improved heterojunction bipolar transistor optoelectronic transimpedance amplifier which has the ability to perform at bit rates greater than 40 Gbits/second.
To achieve the objects and advantages specified above and others, a heterojunction bipolar transistor optoelectronic transimpedance amplifier using the first transistor as an optical detector is disclosed. The optoelectronic transimpedance amplifier is formed on a semiconductor substrate. The transimpedance amplifier contains an input phototransistor for receiving incident optical signals and converting the optical signals to electrical signals.
In the preferred embodiment, the transimpedance amplifier comprises a simple local shunt feedback circuit with an emitter follower. The transimpedance amplifier contains an input transistor and an output transistor. In the preferred embodiment, the input transistor is an edge illuminated waveguide phototransistor that includes a subcollector layer formed from an epitaxially grown quaternary semiconductor material. The epitaxially grown quaternary semiconductor material improves the optical waveguide mode properties. A collector region is epitaxially grown on the subcollector layer. A base region, which contains a very thin spacer layer, is epitaxially grown on the collector layer. An emitter region is then epitaxially grown on the base layer. The various layers and regions are formed so as to define an edge-illuminated facet for receiving incident light. Further, ohmic contacts are formed to the subcollector, base, and emitter regions to allow electrical signals to be applied to the phototransistor.
In a preferred embodiment, the first transistor consists of a semiconductor region that can absorb incident optical radiation centered around wavelengths of 1.55 xcexcm. All of the elements of the transimpedance amplifier circuit are formed on the same substrate, which eliminates the need to use a separate photodetection element. Also, the associated parasitic impedances that arise from connecting the photodetector to the input of the transimpedance amplifier circuit are eliminated. By forming the entire circuit on the same substrate, the amplifier acts as an inexpensive and monolithically integrated front-end optical receiver that is suitable for high-speed optical communication systems.